The art is replete with many types of such handling mechanisms, including apparatus specifically designed for holding workpieces like semiconductor wafers, and transporting them between workstations and or wafer cassettes. Among these are mechanisms using an annular ring member to position a wafer and an annular flange or lip to support the workpieces, as suggested, for example, in U.S. Pat. No. 4,584,045, as part of a dry processing system. U.S. Pat. No. 4,749,330 discloses a simpler version of such a means, eliminating the annular positioning ring. In U.S. Pat. No. 5,334,257, as another example, a paddle works in conjunction with a means to lift the workpiece off a fixed annular ring member. U.S. Pat. Nos. 5,584,647 and 5,810,549 disclose such workpiece holding means as an object handling system.
A disadvantage of these and similar approaches, however, is that the workpiece is not held in a manner that secures the workpiece when the gripper is moved in planes other than horizontal. Another disadvantage is that such apparatus is restricted to picking the workpiece up off a pedestal of some sort, and does not allow for picking the workpiece up off a flat surface, or for picking up the workpiece from above.
A different approach has accordingly been suggested, using a vacuum wand or paddle, wherein a flat upper surface with vacuum ports is brought into contact with the underside of a workpiece and suction is applied at the port to secure the workpiece tot he paddle. U.S. Pat. No. 4,770,590 discloses such a means for use in a transport system. U.S. Pat. Nos. 5,064,030, 5,135,349 and 5,765,444 disclose such a means for use in a precision robot arm system. U.S. Pat. No. 5,803,979 discloses a means of making vacuum workpiece holders more robust against contamination of the workpiece that may clog the vacuum lines. A basic disadvantage of this vacuum approach, however, is that the contact between the paddle and the workpiece can result in microscopic mechanical damage to the workpiece or particle contamination of the workpiece. As microelectronic circuitry evolves to smaller features and higher densities, particle contamination even of the backside of workpieces is undesirable. This technique, furthermore, prohibits picking up the workpiece from its front surface, the surface on which micro-electronic circuits are fabricated.
Another prior art proposal for holding workpieces is the use of a plurality of gripping arms or fingers which are mechanically moved relative to one other to change the distance between contact surfaces which, when brought to their minimal separation, clamp the workpiece in place relative to the arm assembly. These means avoid the previously discussed disadvantage of contacting the work-piece surface, and can be used to pick up a work-piece from a substantially flat surface. U.S. Pat. No. 4,900,214 is illustrative of such proposals, disclosing first and second jaws moveable relative to one other along a diameter of the wafer to be transported. In U.S. Pat. Nos. 5,549,444 and 5,700,046, contacting elements are attached to the inner surface of each gripping member in order to support the workpiece from its bottom surface. U.S. Pat. No. 5,566,466 discloses such a means wherein the gripper mechanism is part of a spindle assembly used to rotate the workpiece while it is clamped by the mechanism.
A disadvantage of such approaches resides in the possibility of particle generation by rubbing between adjacent sliding mechanical surfaces. A further disadvantage is the cost of fabricating and maintaining precision tolerances in an assembly of several parts.
The matter of handling a variety of workpiece sizes has also been considered. U.S. Pat. No. 4,971,512, as an illustration, discloses the use of a fork-shaped workpiece support board that carries the workpiece from the bottom with a moveable arm slide provided to capture the workpiece in its horizontal plane. While such constructions are indeed capable of handling a variety of workpiece sizes, they are unfortunately not useful in applications where workpiece cleanliness on front and back sides is important.
Still another approach to solve these pressing problems makes us of a Bernoulli effect on the top side of the workpiece. U.S. Pat. Nos. 5,080,549 and 5,324,155 disclose such a system wherein a paddle with appropriately placed gas exhaust ports is brought into close proximity with the top side of a workpiece. The exhaust ports are configured so that the gas flow creates a low pressure region on the top side of the workpiece causing it to lift up toward said paddle. The workpiece can therefore be lifted from a substantially flat surface and transported from the top side with no contact with the top side of the workpiece.
A disadvantage of this approach, however, is that it may not be used in a vacuum system. A further disadvantage is that it does not provide for high precision centering of the workpiece; and still another disadvantage in that the holding mechanism restricts the transport motion to paths within a horizontal plane or perpendicular to a horizontal plane.